1. Field of the Invention
The invention relates to a heterojunction bipolar transistor (HBT), and in particular to an HBT with the interesting and different characteristics of amplification and the negative-differential-resistance (NDR) phenomenon under the applied different base current.
2. Description of the Related Art
A conventional HBT is worldwide applied on commerce and military systems. Since a conventional NDR device can simplify circuit design, it is always applied on logic circuits or high-frequency oscillation circuits. When the conventional NDR device is applied on microwave circuits, NDR phenomena are always observed in the large collector current regime. For power transistors, the NDR phenomena are caused by an uneven current distribution and thermal effect on forked electrodes. This results in a lower current gain and unstable characteristics. The NDR phenomenon is rarely observed in the small collector current region. Most disclosed researches considered that the very thin base is a main reason to cause the NDR phenomena. The N-shaped NDR phenomenon is always observed in the large collector current regime. However, both the transistor active regime and NDR loci are observed in the proposed HBT device.
The invention is to provide an HBT with the common-emitter current-voltage (I-V) characteristics of amplification and the NDR phenomena based on the InP/InGaAlAs material system with a conduction band discontinuity of 0.75xcex94Eg (xcex94Eg is the difference of energy gap between InP and InGaAlAs). Besides, a setback layer with a thickness of 50 xc3x85 is added at the emitter-base heterojunction to reduce the potential spike. The xcex4-doped sheet is added at the junction between the emitter and setback layer to cooperate with a base width modulation effect caused by the thin base layer. As a result, the HBT of the presented invention can provide four operation regimes which are significantly different from those of the conventional HBT""s.
The invention is to provide an HBT with a transistor active region for amplification and NDR loci simultaneously. Under large base currents, such as IB=100 xcexcA/step, the current gain up to 28 is obtained. However, under a small base current, such as IB=2 xcexcA/step, an interesting topee-shaped output I-V characteristics is observed. Therefore, both the transistor active region and NDR loci exist simultaneously in the I-V characteristics. Under the applied base current of IB=2 xcexcA/step, the current gain can reach 10. In addition, in the NDR region, the peak-to-valley current ratio is 11. As can be seen from above, under the different applied base currents, the operation of the HBT device can be controlled.
Furthermore, the invention is to provide an HBT with both the transistor active region for amplification and NDR loci. In the structure of the proposed HBT, Al-quaternary material (InGaAlAs) of base and collector layers, which are lattice matched to the InP substrate, are employed. Higher electron mobility and wider modulation of energy gap of InGaAlAs provide a good candidate for high-speed and high-power electronic devices.